Submitted to: Journal of Experimental Botany
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/22/1999
Publication Date: N/A
Citation: Interpretive Summary: 1) Rationale: The goal here was to determine the role of a protein degrading enzyme during low oxygen conditions in maize during flooding. 2) Accomplishments: The results indicate that this enzyme is involved in killing the root tip during flooding. This tissue draws a lot of energy from the rest of the seedling and is not necessary at this stage of the plant's growth. The process appears to be a mechanism to divert energy toward seedling survival during flooding stress. 3) Significance: The understanding of this enzyme's role in a plant's tolerance of low oxygen-stress conditions will allow for a greater understanding of how plants respond to this stress and may allow for effective methods to produce crop plants that are tolerant to flooding.
Technical Abstract: Imposition of anoxia on 3-day old maize seedlings (cv. B73) for 48 h or longer led to the death of the root tip, as indicated by the visual necrotic symptoms, soon after reoxygenation. The necrosis extended into the root axis during post-anoxic growth, leading to the mortality of 30-50% of the seedlings. Using gelatin substrate zymography, we studied protease profiles in the root tip and axis of seedlings submerged for different periods. O2 deprivation for 24 h or longer repressed pre-existing protease activities and induced a novel soluble enzyme in the roots. The anoxia-induced protease (AIP) activity was predominant in the root apex at 24 h of anoxia, and subsequently became the most abundant soluble activity in the root axis as well. The induction of AIP in vivo and its renaturation in vitro was dependent on the availability of Ca2+. Inhibitor sensitivity studies indicated that AIP is a cysteine protease. In SDS-acrylamide gels, the enzyme activity migrated as a 23.5 kD polypeptide. The appearance of the activity in submerged roots was repressed by cycloheximide treatment, suggesting that new protein synthesis was required for the AIP induction. Excision of the root tip before the anoxic treatment (de-tipping) led to a superior recovery of seedlings from stress injury. De-tipped seedlings showed lesser root damage and an increased production of lateral roots compared to intact seedlings. Furthermore, the superior anoxia tolerance of de-tipped seedlings was associated with a decreased AIP activity in their roots. Thus, the appearance of AIP activity in the root tip at 24 h of anoxia was spatially and temporally associated with the initiation and possibly the progression of the root tissue death.